The deflection system of a raster-scan cathode ray tube (CRT) includes a deflection yoke that comprises two pairs of coils mounted at right angles to one another. The coils generate magnetic fields to deflect the electron beam horizontally (along the x-axis) and vertically (along the y-axis) across the screen in response to a video signal.
The video signal is received from an external source (e.g., from a receiver or graphics board) and amplified and applied to the CRT gun to generate a varying-intensity electron beam. The deflection yoke is driven by separate horizontal and vertical deflection circuits. The horizontal circuit supplies a sawtooth-shaped current to the horizontal yoke coils to scan the beam across the CRT, then quickly return it to the other side to begin the next scan line. The vertical deflection circuit performs a similar function with the vertical coils of the yoke, but of course returns the beam at a lower frequency.
The raster-scan CRT also includes a sync separator. To produce a meaningful image the beam must be swept across the screen at the same rate each time, and this must be synchronized so that the rows and columns of pixels that make up the image are arranged as intended. Such synchronization information is provided by the video source in the form of synchronization pulses, or "sync" pulses, which signify the start of a new line or a new frame. These may be separately supplied, e.g., with TTL lines, or may be encoded into the video signal. Most high-resolution displays use such encoded, or composite, video signals.
The standard for such composite video signals (actually one of the more common of several standards) is illustrated in FIGS. 1(a) and 1(b). FIG. 1(a) depicts a portion 10 of a video signal corresponding to one scan line and FIG. 1(b) depicts a portion 12 corresponding to several scan lines. The blanking level, labelled "BLANK (REF.)" in FIG. 1(a), is considered the reference. The display should turn off the beam when a signal at that level is presented. Slightly above the BLANK level (about +0.070 V) is the BLACK level. The BLACK level is the lowest level allowed for the "normal video" range encountered during the "active" portion E of the image. All signal levels between the BLACK level and the reference white level (WHITE), which is 0.714 V above the BLANK level, are expected to be displayable by the monitor.
The onset of a synchronization pulse is indicated when the video signal drops below the blanking level. The level of the sync pulse peaks (labelled "SYNC") are defined to be 0.286 V negative with respect to the blanking level, for an overall signal amplitude (SYNC to WHITE) of IV p--p.
To distinguish between video and sync levels, the display must keep track of the DC blanking level (BLANK), which is difficult to do when the inputs are typically AC coupled. Most designs get around this problem by sampling the signal level just after the sync pulse, when the signal is expected to be at the blanking level, i.e., in the interval D just after the interval C, and then holding this level as a reference. The signal is also expected to be at the blanking level for a short period B before the sync pulse, since blanking both before and after the sync pulse guarantees that the retrace lines will not be visible to the user. These are known as the front porch and back porch of the sync signal, with the entire period F between two lines of visible video called the horizontal blanking interval. There is both a horizontal and a vertical blanking interval specified for any video signal format/timing. In FIG. 1(b), the interval G corresponds to several scan lines, the interval J is the vertical blanking interval, the interval K corresponds to one frame, i.e., one complete image, and the interval M corresponds to the vertical blanking signal. The screen is refreshed at 60 Hz (i.e., each line is scanned once every 16.667 ms) in most modern computer graphics displays. In Europe, with a 50 Hz standard AC line frequency, refresh rates are often 50 Hz.
By far the most exciting advantage of the CRT over other display technologies is its ability to produce images in brilliant, lifelike color. This is particularly important in the computer graphics field, where color is used not only to produce lifelike images, but also to highlight text areas, display attention-getting warnings, etc. Color CRTs make use of the fact that the eye will perceive the proper combination of red, green, and blue light as white, and that other combinations will produce other intermediate colors; thus the color CRT has three guns, one each for red, green, and blue, and three separate phosphors to match.
The field of computer graphics has increased the importance of being able to display high-resolution images. Such images require a higher density of pixels than the 640.times.480 NTSC standard. It is often necessary, however, to convert such high-resolution images to a commonly used format, such as an NTSC, PAL or SECAM-encoded color television signal, for the purposes of display and/or recording via conventional videotape. Unfortunately, presently available devices to perform such conversion (e.g., the Folsom Research card employed by Hewlett Packard Company on its series 300 computers) are typically limited to a narrow range of input signal formats, i.e., they require that the resolution and horizontal and vertical sync rates of the high-resolution image be within a narrow prescribed range. Other devices will convert over a wide range of input signal formats/timings, but those devices place raw digital data obtained by sampling the input video signal into a large frame buffer and then downsample the stored data into a second frame buffer. The use of two frame buffers makes such devices too expensive for many applications.
Accordingly, a primary goal of the present invention is to provide an apparatus and a method for efficiently and inexpensively converting a wide range of high-resolution input signals to a commonly used format. The present invention achieves this goal.